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OrnitJiorliyncTiiis comes nearest to the reptile in the fact that
this axis is nearly at right angles to that of the sacrum. It is
particularly interesting to find that this peculiarity of Orni-
tliorliynclmis is only acquired later in life, and that the pelvis of
the foetus conforms in these angles to the adults of other
mammalian groups. In any case, the backward rotation of the
pelvis is a mammalian characteristic,, and it is most nearly
approached among reptiles by the extinct Anomodontia, whose
affinities to mammals will be dealt with on a later page (p. 90).
Another peculiarity of the mammalian pelvis appears to be the
cotyloid bone already referred to. In the JEttibbit this bone
completely shuts out the pubis from any share in the acetabular
cavity ; later it ankyloses with that bone. In OrnitJiorhi/ncJius
the cotyloid or os acetabuli is a larger element of the girdle
than is the pubis. In other mammals, therefore, it seems to be
a rudimentary structure. But it seems to be a bone peculiar to
and thus distinctive of the mammals as compared with other
vertebrates. The acetabular cavity is perforated in Echidna as
in birds; but in certain Hodents the same region is very thin
and only closed by membrane, as in Circolabes villosus.
The number and the arrangement of the bones in the Mud-limb
correspond exactly to those of the fore-limb. The femur, which
corresponds to the humerus, shows some diversities of form. The
neck, which follows upon the almost globular head, the surface
of articulation to the acetabular cavity of the pelvis, has two
roughened areas or tuberosities for the insertions of muscles, A
third such area, known as the third trochanter, is present or
absent as the case may be, and its presence or absence is of
systematic import. As a general rule the thigh-bones of the
ancient types of mammals are smoother and less roughened by
the presence of these three trochanters than in their modern
representatives. The radius and the ulna are represented in the
hind-leg by the tibia and the fibula. These bones are not
crossed, and do not allow of rotation as is the case with the
radius and the ulna. In Ungulate animals there is the same
tendency to the shortening and rtzdimentary character of the
fibula that occurs in the case of the ulna, but it is more marked.
It has been shown in tracing the history of fossil Ungulates that
the hind-limbs in their degree of degeneration are as a rule
ahead pf the fore-limbs. This is natural when we reflect that